Polymerisation of ethylene homopolymers and copolymers (referred to below as ethylene (copolymers) by radical initiated high pressure polymerisation is well-known in the art. Generally, the polymerisation of the monomer(s) is carried out at a temperature of about 100-300.degree. C. and at a pressure of about 100-300 MPa in the presence of a radical initiator in a polymerisation reactor. Usually the polymerisation is carried out continuously, preferably in a tubular reactor or a stirred tank reactor.
During polymerisation fouling of the reactor may occur, especially when copolymerising ethylene with polar comonomers. The fouling manifests itself as unstable and inhomogeneous production, with formation of gels and build up of polymer deposits on the inner surfaces of the reactor. The gels and polymer deposits, when released from the reactor surfaces will contaminate the final polymer and compromise its quality. Also, the unstable production due to fouling makes it difficult to produce a polymer with a consistent and reproducible quality. Although reactor fouling may occur when producing ethylene homopolymers as well as copolymers, as mentioned above, it is particularly pronounced in connection with polymerisation of copolymers of ethylene and polar comonomers which polymerise more easily than ethylene. By way of example such polar comonomers comprise .alpha.,.beta.-unsaturated carboxylic acids having 3-8 carbon atoms, anhydrides thereof, or esters thereof with aliphatic alcohols having 1-8 carbon atoms, such as (meth)acrylic acid, methyl(meth)acrylate, ethyl(meth)acrylate, and butyl(meth)acrylate. The expression "(meth)acrylic acid" includes both acrylic acid and methacrylic acid. Similarly, "alkyl(meth)acrylate" includes alkyl acrylates as well as alkyl methacrylates. Other polar comonomers such as vinylesters of saturated carboxylic acids having 1-4 carbon atoms and in particular vinyl acetate are less prone to cause problems with reactor fouling. This is probably due to their lower reactivity which usually leaves some of the comonomer in the polymerisation mixture as a solvent. However, when making high molecular weight copolymers reactor fouling is encountered also with this type of comonomer. Examples of polar comonomers that normally do not cause fouling problems are unsaturated silane compounds, such as vinyl trimethoxy silane, vinyl triethoxysilane, gamma-(meth)acryloxypropyltrimethoxysilane and gamma-(meth)acryloxypropyltriethoxysilane.
The above-mentioned reactor fouling problem is more pronounced the higher the amount of polar comonomer is. Generally, fouling starts to be a problem already at a comonomer content of about 15% by weight, and at a comonomer content of about 20 to 25% by weight it is difficult to carry out polymerisation and recover the polymer due to heavy fouling of the reactor and contamination of the polymer. Thus, when polymerising ethylene and ethyl acrylate problems with fouling and unstable production start at about 15-16% by weight of ethyl acrylate and get increasingly more serious up to about 25% by weight of ethyl acrylate when fouling makes production almost impossible. Similarly, it is not possible to copolymerise ethylene and methyl acrylate at higher methyl acrylate contents than about 20% by weight.
It has been suggested to solve the problem of reactor fouling in different ways, e.g. by periodic reactor scraping and addition of additives such as adhesion inhibitors. According to EP-A-0,460,936 it is e.g. proposed to solve the problem by introducing into the copolymerisation reactor a solvent consisting essentially of methanol in an amount of 2-25% by weight of the total material flowing through the reactor. A disadvantage with the addition of methanol is that it acts as a chain transfer agent and reduces the molecular weight of the copolymer.
Although the solutions proposed by the prior art may alleviate the fouling problem to some degree, so far no truly effective solution has been disclosed. Because of the seriousness of the problem, an effective solution to the reactor fouling problem would be an important advantage both from a technical and economical point of view.